Solid polymer electrolytes are highly attractive alternatives to conventional liquid electrolytes in energy conversion devices such as dye-sensitized solar cells and electrochromic windows, and energy storage devices such as lithium ion batteries and super capacitors, because of issues related to device fabrication and safety.
Dye-sensitized solar cells (DSSCs) have generated considerable interest in recent years, because they can be fabricated at a relatively low-cost, and are suitable for incorporation in flexible substrates using roll-to-roll processing. In addition, DSSCs using Ru-complex sensitizers have power conversion efficiencies as high as 11%, which makes these cells promising cost-effective alternatives to the classical crystalline silicon cells.
In a typical DSSC, titanium dioxide nanoparticles are deposited onto a conductive glass substrate and heated to form a nanoporous coating. The coated substrate is then dipped into a solution of ruthenium containing dye, in order to absorb the dye molecules on TiO2 surface. The pores of the TiO2 coating are further infused with a liquid electrolyte that contains iodide salts. The device fabrication is completed by placing a counter electrode of platinum-coated conductive glass on top of this nanoporous TiO2 coating. Upon absorption of light, the sensitizer molecules inject electrons into the TiO2, and are oxidized. The iodide, I−, in the electrolyte supplies electrons to the oxidized dye, and regenerates the dye. The iodide oxidizes to triiodide, I3−, during this step. The electrons passing through an external load arrive at the counter electrode (cathode) and reduce the triiodide back to iodide. It is evident that the photo-current density, and the power output of the solar cell, will increase with an increase in the mobility of the redox mediators (I3−/I−) in the electrolyte solution.
High power conversion efficiencies (11%) in DSSCs have been obtained using liquid electrolytes, such as acetonitrile, that support high ionic conductivities [See, Chiba. Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Dye-sensitized solar cells with conversion efficiency of 11.1%, Japan. J. Appl. Phys. 2006, 45, L638-L640 incorporated herein by reference]. However, conventional (often volatile) liquid electrolytes limit the long-term stability and high temperature operation of DSSCs. Moreover, thermal expansion results in leakage of liquid electrolytes in DSSCs operated at elevated temperatures. Other problems in conventional DSSCs include desorption of dye molecules from the TiO2 surface, and chemical degradation of the catalytic features of the TiO2/Pt electrodes. The development of DSSCs for commercial applications largely relies on successful resolution of these limitations. The organic electrolytes of the present invention overcome the drawbacks of liquid electrolytes. They eliminate problems related to sealing and evaporative loss of solvent.
Organic electrolytes have been explored by several researchers as materials for dye-sensitized solar cells. Salminen et al. have reported physicochemical and biological properties of a variety of piperidinium and pyrrolidinium ionic liquids [See. Salminen, J.; Papaiconomou, N.; Kumar, R. A.; Lee, J.-M.; Kerr, J.; Newman, J.; Prausnitz, J. M. Physicochemical properties and toxicities of hydrophobic piperidinium and pyrrolidinium ionic liquids, Fluid Phase Equilibria 2007, 261, 421-426 hereby incorporated herein by reference]. Properties of imidazolium and pyridinium ionic liquids have also been reported [See, Papiconomou, N.; Yakelis, N.; Salminen, J.; Bergman, R.; Prausnitz, J. M. Synthesis and properties of seven ionic liquid containing 1-methyl-3-octylimidazolium or 1-butyl-4-methylpyridinium cations, J. Chem. Eng. Data 2006, 51, 1389-1393; and Papiconomou, N.; Salminen, J.; Lee, J.-M.; Prausnitz, J. M. Physicochemical properties of hydrophobic ionic liquids containing 1-octylpyridinium, 1-octyl-2-methylpyridinium, or 1-octyl-4-methylpyridinium cations, J. Chem. Eng. Data 2007, 52, 833-840 both hereby incorporated herein by reference]. However, none of these ionic liquids contained iodide anions.
Kawano et al. have reported the use of ionic liquid crystal electrolytes for construction of dye-sensitized solar cells [See, Kawano, R.; Nazeeruddin, M. K.; Sato, A.; Grätzel, M.; Watanabe, M. Amphiphilic ruthenium dye as an ideal sensitizer in conversion of light to electricity using ionic liquid crystal electrolyte, Electrochem. Commun. 2007, 9, 1134-1138 hereby incorporated herein by reference]. Liquid blends of different ionic liquids have also been used as electrolytes for DSSCs [See, Kuang, D.; Klein, C.; Zhang, Z.; Ito, S.; Moser, J.-E.; Zakeeruddin, S. M.; Gratzel, M. Stable, high-efficiency ionic-liquid-based mesoscopic dye-sensitized solar cells, Small 2007, 3, 2094-2102, hereby incorporated herein by reference]. Gelation agents were used with 1-alkyl-3-methylimidazolium iodides to prepare gel electrolytes for DSSCs [See, Kubo. W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Photocurrent-determining processes in quasi-solid-state dye-sensitized solar cells using ionic gel electrolytes, J. Phys. Chem. B 2003, 107, 4374-4381 hereby incorporated herein by reference]. While all of these electrolytes contained the iodide anion, none of them contained the ethylene oxide, —CH2CH2O—, or perfluoroethyl, —CF2CF2— groups.
Freitas et al. have reported the use of polymer electrolytes based on mixtures of poly(ethylene oxide-co-propylene oxide) and 1-propyl-3-methylimidazolium iodide (PrMImI) in dye-sensitized solar cells [See, Freitas, F. S.; de Freitas, J. N.; Ito, B. I.; De Paoli, M.-A.; Nogueira, A. F. Electrochemical and structural characterization of polymer gel electrolytes based on a PEO copolymer and an imidazolium-based ionic liquid for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 2009, 1, 2870-2877 hereby incorporated herein by reference]. Of the different iodide salts that form room-temperature ionic liquids, PrMImI has been a candidate of choice for DSSCs because it has the lowest viscosity [See. Bai. Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nature Mater. 2008, 7, 626-630 hereby incorporated herein by reference]. A blend containing 70 wt % of PrMImI showed the highest ionic conductivity (2.4×10−3 S/cm). In the electrolytes used by Freitas et al., the ethylene oxide groups were not covalently attached to the imidazolium cation. The liquid electrolyte, PrMImI, was present within a non-ionic polymer host matrix [poly(ethylene oxide-co-propylene oxide)]. To achieve high ionic conductivity values, 70 wt % of the polymer electrolyte consisted of the liquid component, PrMImI. Similar blends of PrMImI, or 1-ethyl 3-methylimidazolium thiocyanate, with poly (ethylene oxide) have been reported by Singh et al. [See, Singh, P. K.; Kim, K.-W.; Rhee, H.-W. Ionic liquid (1-methyl 3-propyl imidazolium iodide) with polymer electrolyte for DSSC application, Polym. Eng. Sci. 2009, 49, 862-865; Singh, P. K.; Kim, K.-I.; Park, N.-G.; Rhee, H.-W. Dye sensitized solar cell using polymer electrolytes based on poly(ethylene oxide) with an ionic liquid. Macromol. Symp. 2007, 249-250, 162-166 hereby incorporated herein by reference]. The ionic liquid resulted in a small reduction in the crystallinity of poly(ethylene oxide), which led to about 2-fold increase in the ionic conductivity. The electrolytes of the present invention, on the contrary, contain organic electrolytes with fluoroalkyl-tagged PEG groups, covalently attached to imidazolium cation. Moreover, these ion conducting solid electrolytes are blended with another solid electrolyte 1-ethyl-3-methylimidazolium iodide (EtMImI) to achieve high ionic conductivities (1.11×10−4 S/cm at 30° C. and 2.88×10−3 S/cm at 90° C.
Kang et al. prepared PEGylated polymer electrolytes using three different approaches. [See, Kang, M.-S.; Kim, J. H.; Won, J.; Kang, Y. S. Oligomer approaches for solid-state dye-sensitized solar cells employing polymer electrolytes, J. Phys. Chem. C 2007, 111, 5222-5228 hereby incorporated herein by reference]. In the first approach, supramolecules of PEG oligomer having two quadruple hydrogen bonding sites at both chain ends were used. The formation of quadruple hydrogen bonds, upon evaporation of the solvent used for casting the electrolyte film, resulted in solidification of the electrolyte. In the second approach, an oligomer blend containing amorphous liquid oligomer poly(propylene glycol) was used with high molecular weight poly(ethylene oxide). In the third approach, low molecular weight poly(ethylene glycol) oligomers were solidified using fumed silica nanoparticles to obtain solid polymer electrolyte nanocomposites. In all three approaches, ionic conduction was achieved by adding potassium iodide or the 1-propyl-3-methylimidazolium iodide (PrMImI) ionic liquid. Similar quasi-gel-electrolytes, based on PrMImI have been prepared using biocatalytically synthesized PEGylated polymers [See, Kumar, R.; Sharma, A. K.; Parmar, V. S.; Watterson, A. C.; Chittibabu, K. G.; Kumar, J.; Samuelson, L. A. Flexible, dye-sensitized nanocrystalline solar cells employing biocatalytically synthesized polymeric electrolytes, Chem. Mater. 2004, 16, 4841-4846 hereby incorporated herein by reference]. In contrast, the electrolytes of the present invention are not polymer gel electrolytes, but are solids or high viscosity materials that can function in a solid DSSC without a polymer matrix or an inorganic support.
Dicationic bis-imidazolium iodides have been used in the construction of DSSCs, but these electrolytes are liquids at room temperature, with low viscosity [See, Zafer, C.; Ocakoglu, K.; Ozsoy, C.; Icli, S. Dicationic bis-imidazolium molten salts for efficient dye sensitized solar cells. Synthesis and photovoltaic properties. Electrochmica Acta 2009, 54, 5709-5714 hereby incorporated herein by reference]. The electrolytes contained the triethylene glycol and the tetraethylene glycol groups, but did not contain fluoroalkyl groups (FIG. 1). The present invention pertains to the use of relatively high viscosity liquids, or solid PEGylated and fluorinated imidazolium iodides, in combination with solid electrolytes such as 1-ethyl-3-methyl imidazolium iodide (EtMImI).
FIG. 1 illustrates ionic liquid electrolytes for DSSC [See, Zafer et al., 2009, ibid.].
Bai et al. found that a mixture of 1-allyl-3-methylimidazolium iodide (AMImI), 1,3-dimethylimidazolium iodide (DMImI), and 1-ethyl-3-methylimidazolium iodide (EtMImI), in a molar ratio of 1:1:1, was a ternary melt with a melting point below 0° C. The ternary melt, with enhanced fluidity, exhibited a room temperature conductivity of 1.68×10−3 S/cm. In contrast, the EtMImI blends of the present invention were not low-viscosity fluids at room temperature, and yet showed significantly improved ionic conductivity (relative to controls without EtMImI). Amorphous blends, as well as those that contained crystalline phases of EtMImI, showed high ionic conductivity.
Wang et al. have reported polymer electrolytes containing chemically crosslinked gelators [See, Wang, L.; Fang, S.-B.; Lin, Y. Novel polymer electrolytes containing chemically crosslinked gelators for dye-sensitized solar cells, Polym. Adv. Technol. 2006, 17, 512-517 hereby incorporated herein by reference]. These electrolytes consisted of a polypyridyl-pendant dendritic derivatives, and multifunctional halogen derivatives of PEG (FIG. 2). Although, these polymers are halogenated (chlorinated), they do not contain the perfluoroalkyl, —(CF2CF2)yF, group. Moreover, these high molar mass polymers (Mn=70,000 g/mol) will be unable to penetrate the pores of TiO2 efficiently.
FIG. 2 illustrates a high molecular weight multifunctional halogen derivative of PEG [See, Wang et al., 2006, ibid.].
Similarly, Shim et al. have recently reported the use of poly(imidazolium iodide) of the structure shown in FIG. 3, which are not fluorinated [See, Shim, H. J.; Kim, D. W.; Lee, C.; Kang, Y. In situ crosslinked ionic gel polymer electrolytes for dye sensitized solar cells. Macromol. Res. 2008, 16, 424-428 hereby incorporated herein by reference]. Moreover, in situ formation of the polymers of Shim et al. required heating the reactive components at 100° C. for 21 h. The conducting iodide anions are formed only during the high temperature curing process (after the precursors of the polymer electrolyte have been injected into the DSSC device). In contrast, the organic electrolytes of the present invention do not require thermal processing for extended time periods, after application on the device.
FIG. 3 illustrates chemical structure of polymer electrolyte reported by Shim et al. [Shim et al., 2008, ibid.]
Quasi-solid dye-sensitized solar cells have been fabricated by surface modification of nanopores in an alumina film, and filling these nanopores with ionic liquid electrolytes [See, Takeshi, K.; Hayase, S.; Kaiho, T.; Taguchi, M. Quasi-solid state dye sensitized solar cells having straight ion paths, J. Electrochem. Soc. 2008, 155, K166-K169 hereby incorporated herein by reference]. In contrast, the electrolytes of the present invention can be used even without the porous membrane support.
Polysilsesquioxane polymers prepared using an imidazolium iodide derivatized with covalent attachment of trimethoxysilane group have been used as electrolytes for DSSCs [See, Stathatos, E.; Jovanovski, V.; Orel, B.; Jerman, I.; Lianos, P. Dye-sensitized solar cells made by using a polysilsesquioxane polymeric ionic fluid as redox electrolyte, J. Phys. Chem. C 2007, 111, 6528-6532; and Orel, B.; Jese, R.; Vuk, A. S.; Jovariovski, V.; Perse, L. S.; Zumer, M. Structural studies of trimethoxysilane containing R′R″Im+I− ionic liquid and its nanocomposite with tetramethoxysilane (TMOS), J. Nanosci Nanotech. 2006, 6, 382-395 both hereby incorporated herein by reference]. The electrolytes of the present invention do not contain siloxane groups.
Jiang and Fang have reported the synthesis and physicochemical characterization of PEGylated imidazolium iodides, but the use of these electrolytes in dye-sensitized solar cells was not proposed [See, Jiang, J.; Fang, S. New composite polymer electrolytes based on room temperature ionic liquids and polyether, Polym. Adv. Technol. 2006, 17, 494-499 hereby incorporated herein by reference].
Lee et al. have reported a polymer gel electrolyte consisting of a blend of a high molecular weight poly(ethylene oxide) (Mw˜5×106 g/mol; 47.2 wt % in blend), which is a solid at room temperature, a low molecular weight poly(ethylene glycol) (Mw˜200 g/mol; 31.5 wt %), which is a liquid at room temperature, and EtMImI (Mw238.07 g/mol; 21.3 wt %), to obtain a solid electrolyte with maximum ionic conductivity of 9.2×10−5 S/cm [See, Lee, J. Y.; Bhattacharya, B.; Kim, Y. H.; Jung, H.-T.; Park, J.-K. Self degradation of polymer electrolyte based dye-sensitized solar cells and their remedy, Solid State Commun. 2009, 149, 307-309 hereby incorporated herein by reference]. On the contrary, blends of the fluorinated imidazolium iodide salts of the present invention do not contain any liquid component (e.g. PEG), and yet show a conductivity of at least 1.11×10−1 S/cm at 30° C. and 2.88×10−3 S/cm at 90° C.
Polymer gel electrolytes consisting of solid poly(acrylic acid) matrix, and liquid poly(ethylene glycol) have been reported by Wu et al. [See, Wu, J.; Lan, Z.; Lin, J.; Huang, M.; Hao, S.; Sato, T.; Yin. S. A novel thermosetting gel electrolyte for stable quasi-solid-state dye-sensitized solar cells. Adv. Mater. 2007, 19, 4006-4011 hereby incorporated herein by reference]. The gel was loaded with a liquid electrolyte solution consisting of γ-butyrolactone, N-methyl pyrrolidone, sodium iodide and iodine, and used in the fabrication of a dye-sensitized solar cell. The electrolytes of the present invention do not need liquid organic solvents, or inorganic salts (such as NaI or LiI), to exhibit ionic conductivity.